fix: let a project file ask whether it is the top-level project

Crafter.LintRules declares a test that links crafter-build, fetched
with GitProject. crafter-build's own project.cpp loads Crafter.LintRules
as a `#pragma crafter dependency`, so the chain was: crafter-build loads
LintRules, LintRules' test wiring loads crafter-build's project.cpp from
the external cache, which loads LintRules again, and the cycle guard
threw. Every command that loads project.cpp in this repository has
failed since that test was added, including the last CI run on master.

A project file had no way to tell whether it was the one the user
pointed crafter-build at or a dependency being loaded for somebody
else, so it could not skip wiring that only its own build needs.
IsTopLevelProject() answers that from the loading-frame stack
ProjectDependency() already reads: one frame is the top-level project
file, a second is only ever pushed for a dependency. Outside any project
file it is true as well, which is what a test harness constructing
Configurations expects.

The ProjectDependency test records the answer from both the app and the
greeter project files and checks each.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

interfaces/Crafter.Build-Platform.cppm

@@ -41,6 +41,15 @@ namespace Crafter {
     // Throws when no such dependency was declared, or when called from
     // anywhere other than inside LoadProject.
     export CRAFTER_API Configuration* ProjectDependency(std::string_view name);
+    // True while the project file crafter-build was pointed at runs its
+    // CrafterBuildProject, and when no project file is running at all; false
+    // inside a dependency's project file (a `#pragma crafter dependency`,
+    // GitProject or LocalProject target). Wire anything only the project's
+    // own build or test run needs behind it. The case that forced it: a
+    // library's test links crafter-build itself, so when crafter-build loads
+    // that library as a dependency, the test's GitProject loads crafter-build's
+    // project file in turn and the two recurse until the cycle guard throws.
+    export CRAFTER_API bool IsTopLevelProject();
     // Resolves the directory holding distributed runtime assets (Crafter.Build
     // module sources, wasi-runtime/, etc). Honors CRAFTER_BUILD_HOME; otherwise
     // derives <prefix>/share/crafter-build from the running executable's path.